Dichroism, infrared

The detailed information about molecular orientation in oriented film specimens can be obtained from IR dichroism measurements (Hu et al., 1985 Wu et al., 1986). Light is absorbed when the direction of oscillation of its electrical vector has a component along the direction of oscillation of the absorbing group. The intensity of the absorption band of an oriented polymer thus depends on the direction of the electrical vector of the incident ray 

Using this technique Wen and coworkers (Wen et al., 1988) studied the molecular orientation in the nematic phase of the liquid crystalline polyester with following repeating units  

From the dichroic ratio R of the phenyl ring C—H out-of-plane bending bands (870 and 720 cm-1), the transition moment of which is perpendicular to the long axis of the chain and the mesogenic units, the order parameter was found to be 0.45. 

IR dichroism is also used to obtain the orientation of mesogenic units relative to the main chain in side-chain type liquid crystalline polymers. One example was given by Xu et al. (1993). The polymer studied by them was a mesogen-jacketed liquid crystalline polymer shown below  

In this study two IR bands, 2905 cm 1 and 2939 cm 1 were used to discuss the orientation of the main-chain axis of the polymer molecules. They were 

Pedicini and Farris (2003) used the dichroic ratio of the — NH stretching band (3320 cm ) in polyurethane nanofibers electrospun from 7% DMF to estimate the orientation function / for nanofibers. Herman s function, /, varies from/= 0 (isotropic) to/= 1 (perfect chain orientation). The value of/is related to the average angle 6 made by the oriented chain axis to that of the fiber (Fig. 9.13)  

The dichroic ratio D is the ratio of the absorbance of polarized light measured with the electric vector direction of the polarizer oriented paraUel to the fiber draw direction, and that corresponding to the perpendicular orientation, A. (i.e., D = Ay/A ). Then,/can be expressed in terms of D (Pedicini and Farris 2003) as follows  

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If orientation is assumed to occur only in one dimension (an oversimplification), birefringence and several related phenomena (infrared dichroism, etc.) measure the quantity , which is the average angle 8 between the molecular chain direction and that of the orienting force, such as the fiber stretch axis. It is convenient to introduce an orientation function [12]... 

Beer, M. Quantitative interpretation of infrared dichroism in partly oriented polymers. Proc. roy. Soc. A236, 136—140 (1956). 

Elliott, A. Infrared dichroism of synthetic polypeptides. Nature (Lond.) 172, 359—360 (1953). 

Interpretation of infrared dichroism in axially oriented polymers. J. chem. Phys. 28, 1113—1115 (1958). 

An example of a relevant optical property is the birefringence of a deformed polymer network.256 This strain-induced birefringence can be used to characterize segmental orientation, and both Gaussian and non-Gaussian elasticity.92,296-302 Infrared dichroism has also been particularly helpful in this regard.82,303 In the case of the crystallizable polysiloxane elastomers, this orientation is of critical importance with regard to strain-induced crystallization, and the tremendous reinforcement it provides.82... 

Linear viscoelastic measurements using infrared dichroism on the compatible blend polyethylene oxide) and poly(methyl methacrylate) were reported by Zawada et al. [139]. Unlike Monnerie and coworkers [127], who reported seeing only orientation in the PMMA component, and none in the PEO, Zawada et al. observed alignment in the PEO. However, since the PEO was of lower molecular weight (as was the case for Monnerie and coworkers), its relaxation timescales were substantially faster than the PMMA. This may explain the lack of any measurable orientation by Monnerie and coworkers, who studied quenched samples, since their preparation may have allowed the PEO to relax prior to testing. 

Case Study 2 Dynamics of Multicomponent Polymer Melts - Infrared Dichroism 213 tures become oriented in the flow direction. 

In this case study, which is extracted from reference 184, infrared dichroism is described as a means of separating the component dynamics in multicomponent polymer melts. What is necessary is the existence of distinct absorption peaks for at least one of the components. In the present problem, however, where two chains of identical chemistry but different molecular weights are mixed, there will not be any intrinsic differences in their absorption spectra. In this case it is necessary to label one of chains with a tag that will allow its presence in the blend to be revealed. For this purpose, deuteration of one of the chains is often used. This provides the labeled chain with an absorption of infrared light at the symmetric stretching vibration of the C-D bond, which occurs in the vicinity of 2180 cm-1. Fortunately, the unlabeled polymer contains no absorption peak at this location. It is important, however, to determine that the presence of a label on one species will not alter the physical response of the sample at a level that will affect the phenomena under study. For example, the labeling should not induce phase separation or cause unwanted specific interactions. 

B. Amram, L. Bokobza, P. Sergot, and L. Monnerie, Effect of temperature on intermolecular orientational correlations between chain segments in strained poly-isoprene A Fourier transform infrared dichroism investigation, Macromolecules, 23, 1212 (1990). 

J.A. Komfield, G.G. Fuller, and D.S. Pearson, Infrared dichroism measurements of molecular relaxation in binary blend melt rheology, Macromolecules, 22,1334 (1989). 

The results discussed in this chapter demonstrate that 2H NMR is a powerful technique for investigating microscopic properties in rubber networks. Most of the experiments described here are easy to handle on standard NMR equipment. Due to the absence of interactions between 2H nuclei, spectra and line shapes are easy to interpret and give quite direct information, at least in the first step of analysis, which is that generally required to correlate microscopic to macroscopic properties in these systems. Additionally, in contrast to optical techniques (as birefringence, infrared dichroism, fluorescence polarisation) the information which is obtained is very specific, because spatial and temporal averaging processes are clearly distinguishable in NMR. 

Reisdorf WC, Krimm S. Infrared Dichroism of amide I and amide II modes of a - and atn-hclix segments in membrane proteins. Biophys I 1995 69 271-273. 

Chain relaxation processes of uniaxially stretched polymer chains an infrared dichroism study... 

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